1. Field of the Invention
The present invention relates to an apparatus and method for transmitting and receiving an Acknowledge/Negative acknowledge (ACK/NACK) signal supporting Hybrid Automatic Repeat reQuest (HARQ) in a mobile communication system.
2. Description of the Related Art
Currently, in mobile communication systems, intensive research is being conducted on Orthogonal Frequency Division Multiple Access (OFDMA) or Single Carrier-Frequency Division Multiple Access (SC-FDMA) as a potential scheme for high-speed data transmission on wireless channels.
3rd Generation Partnership Project (3GPP), a standard group for asynchronous cellular mobile communication, is studying Long Term Evolution (LTE) or an Evolved Universal Terrestrial Radio Access (E-UTRA) system, which is the next-generation mobile communication system, based on the above-stated multiple access scheme.
The multiple access scheme allocates and manages time-frequency resources on which data or control information is transmitted for each user without overlapping each other, i.e., orthogonality is maintained for the time-frequency resources in order to distinguish data or control information of each user. For a control channel, the multiple access scheme can additionally allocate code resources for distinguishing control information of each user.
FIG. 1 is a diagram illustrating a transmission structure on a time-frequency domain for data or control channels transmitted over a DownLink (DL) in an LTE system to which the present invention is applied.
In FIG. 1, the vertical axis represents a time domain, and the horizontal axis represents a frequency domain. The minimum transmission unit in the time domain is an OFDM symbol. Nsymb OFDM symbols 102 are included in one slot 106 and two slots are included in one subframe. A length of the slot is 0.5 ms, and a length of the subframe is 1.0 ms. The minimum transmission unit in the frequency domain is a subcarrier, and the entire system transmission band includes a total of NBW subcarriers 104.
In the time-frequency domain, the basic unit of wireless resources is a Resource Element (RE) 112, which can be represented by an OFDM symbol index and a subcarrier index. A Resource Block (RB) 108 is defined by Nsymb consecutive OFDM symbols 102 in the time domain, and NRB consecutive subcarriers 110 in the frequency domain. Therefore, one RB 108 includes Nsymb*NRB REs 112. Generally, the minimum transmission unit of data is the RB. In the current LTE system, Nsymb=7, NRB=12, and NBW has a value that is proportional to the system transmission band.
It is assumed that control information is transmitted within first N OFDM symbols in a subframe. Presently, a maximum of 3 is considered as a value of N. Therefore, a value of N varies according to the amount of control information to be transmitted on a subframe.
The control information includes an indicator of the number of OFDM symbols over which the control information is transmitted, UpLink (UL) or DL scheduling information, an ACK/NACK signal, and Multiple Input Multiple Output (MIMO)-related control information.
HARQ is an important technology used for increasing reliability and data throughput of data transmission in a packet-based mobile communication system. HARQ refers to a combined technology of an Automatic Repeat reQuest (ARQ) technology and a Forward Error Correction (FEC) technology.
ARQ refers to a technology in which a transmitter assigns sequence numbers to data packets according to a predetermined scheme and transmits the data packets. A receiver requests the transmitter to retransmit missing packet(s) among the received packets using the sequence numbers, thereby achieving reliable data transmission.
FEC refers to a technology for adding redundant bits to transmission data before transmission, such as the convolutional coding or turbo coding, to cope with an error occurring in the noise or fading environment during the data transmission/reception process, thereby decoding the originally transmitted data.
In a system using HARQ, a receiver decodes received data through an inverse FEC process, and determines if the decoded data has an error through Cyclic Redundancy Check (CRC) check. If there is no error, the receiver feeds back an ACK to the transmitter, so that the transmitter can transmit the next data packet. However, if there is an error, the receiver feeds back a NACK to the transmitter, thereby requesting retransmission of the previously transmitted packet. Through the above process, the receiver combines the previously transmitted packet with the retransmitted packet, thereby obtaining energy gain and improved reception performance.
FIG. 2 is a diagram illustrating an example of data transmission by HARQ to which the present invention is applied.
Referring to FIG. 2, the horizontal axis represents the time domain. Reference numeral 201 represents an initial data transmission. A data channel is a channel over which data is actually transmitted. A receiver, receiving data transmission 201, attempts demodulation on the data channel. In this process, if it is determined that the data transmission fails in successful demodulation, the receiver feeds back a NACK 202 to a transmitter. Upon receipt of the NACK 202, the transmitter performs retransmission on the initial transmission 201, i.e., a first retransmission 203. Therefore, data channels in the initial transmission 201 and the first retransmission 203 transmit the same information. Even though the data channels transmit the same information, they may have different redundancies.
Upon receipt of the data transmission 203, the receiver performs combining on the retransmission 203 with the initial transmission 201 data, and attempts demodulation of the data channel depending on the combining result. If it is determined that the data transmission fails to successfully demodulate, the receiver feeds back a NACK 204 to the transmitter. Upon receipt of the NACK 204, the transmitter performs a second retransmission 205, a predetermined time period after the time of the first retransmission 203. Therefore, data channels for the initial transmission 201, the first retransmission 203, and the second retransmission 205 all transmit the same information.
Upon receiving data of the second retransmission 205, the receiver combines the initial transmission 201, the first retransmission 203, and the second retransmission 205, and demodulates the data channel. If it is determined that the data transmission is successfully demodulated, the receiver feeds back an ACK 206 to the transmitter.
Upon receipt of the ACK 206, the transmitter performs another initial transmission 207 on the next data. The initial transmission 207 can be immediately performed when the ACK 206 is received, or can be performed after a lapse of a certain time, depending on the scheduling result.
In order to support HARQ, the receiver should transmit an ACK/NACK, or feedback information, to the transmitter. A channel used for transmitting the ACK/NACK is called a Physical HARQ Indicator Channel (PHICH).
When such communication environments are taken into consideration, there is a need for a detailed description as to how the system using HARQ will transmit an ACK/NACK signal in connection with data transmission. In particular, there is a need for a detailed scenario as to how an FDMA-based mobile communication system will transmit ACK/NACK signals for a plurality of users within first N OFDM symbols in a subframe, i.e., there is a demand for an ACK/NACK signal transmission and reception scheme in which HARQ is supported and orthogonality is guaranteed for a plurality of users in the time-frequency domain.